Why would a battery die overnight?
If this is a one-off event, you likely left a light on. When your car is off, the battery stores the charge your vehicle needs to start again. But if you leave an interior light, headlight, or radio on after shutting the vehicle off, it will use all of your battery’s juice and your car won’t turn on the next day. To fix this, simply jump your battery and be more careful about shutting everything off in the future.
If it happens multiple times, you either have a bad battery or a parasitic drain. If your battery keeps dying overnight, you almost certainly have one of two conditions impacting your car: Bad battery: If the car battery itself is no good, you may be able to jump the vehicle just fine because the alternator takes over the power duties after the car has started. Replacing the battery will solve this problem. Parasitic Drain: A parasitic drain refers to some component or accessory that remains on after you’ve shut the car off. In this case, locating the parasitic drain and fixing the underlying issue will keep your car from dying overnight.
Ruling Out the Battery
Inspect the battery to see if it’s swollen or corroded. A swollen battery is on the verge of explosion and must be replaced immediately by a professional mechanic. If you have corrosion, it’s a sign your battery is on the verge of failing and needs to be replaced soon. In either case, a new battery should solve your battery-dying woes. In the future, know that your battery needs to be replaced every 3-5 years.
Use the headlight test to check for a parasitic draw. If you don’t have a multimeter, perform a load test with your headlight. Put the car in the accessory position and turn your headlights on. Wait 10-15 minutes and then start your car. Here are what the results mean: If your headlights dim as the engine starts… Your battery is not stable and it needs to be replaced. The battery shouldn’t be able to hold a steady charge for 10-15 minutes without draining, so if it can’t stay stable, it’s time for a new battery. If your headlights don’t dim or flicker… your battery is fine. It’s storing its charge perfectly well and is unlikely to be the problem. You may still have a parasitic draw, though.
Diagnosing Parasitic Draws
Test the battery with a multimeter to confirm you have a draw. Set the multimeter to milliamps. Keep the vehicle off and disconnect the negative battery terminal. Plug the black (negative) lead into the neutral common (COM) port on the multimeter. Set the red (positive) probe in the amps input. Connect the red probe on the multimeter to disconnected negative battery cable. Then, hook the black probe of the multimeter up to the negative battery terminal. If you have a parasitic drain, the multimeter dial will react. The car must be completely off and the battery must be fully charged for this to work. Key note on the dial moving: If the dial pops up a little then drops below 25mA, you do not have a parasitic draw. Some vehicles maintain a tiny charge while the car is off, so a tiny reading is no big deal. If your multimeter doesn’t react, your batter is not draining when the car is off and you can safely rule out parasitic draws as the problem. If your battery continues to die overnight anyway, your battery is bad and needs to be replaced.
Access the fuse box and keep the multimeter where it is. Keep the multimeter attached to the negative battery cable and negative terminal. Locate the fuse box in your vehicle and open the cover. If you cannot find your fuse box, refer to your instruction manual. Your fuse box is typically located either inside of the car, below the steering wheel, or inside of the engine bay just behind the steering wheel.
Remove the fuses one by one and check the multimeter. You’ve located the source of the draw once the multimeter drops below 25mA. Start with any fuse in the box. Remove the fuses one by one until your battery stops drawing power.
Fix the underlying issue based on whatever fuse is to blame. The fuse that you removed corresponds to a specific electrical component or accessory. You have one fuse for your headlights, one for the radio, and one for every other single electrical system in the vehicle. Refer to your manual’s fuse diagram to see which component is to blame and either replace or repair the issue. If you cannot personally fix the problem (which is likely, since electrical components are a serious pain to work on if you aren’t a licensed mechanic), just see a professional and explain what’s going on. They’ll help you out. Just to clarify, the fuse you removed that caused the multimeter to drop below 25mA is the fuse that corresponds to the component responsible for the parasitic drain. The fuse itself is perfectly fine.
Other Potential Causes
Faulty Alternator If your battery is dying overnight when the vehicle isn’t running, it’s extremely unlikely that the alternator is to blame. Most of the time, a faulty alternator will cause the battery to die after the car starts—not when it’s off. Still, a buggy alternator may not recharge a battery properly and cause the battery to drain after turning the car off. Your alternator is likely to be to blame if you notice weird electrical surges when you’re driving (i.e. the headlights dim and brighten or the radio gets quiet and then loud) or if your car stalls out and dies while you’re driving.
Bad voltage regulator The voltage regulator is responsible for measuring the input and output of all electrical systems in your vehicle. If the voltage regulator is damaged or failing, your vehicle’s electrical systems won’t function properly and you’re likely to encounter a dead battery every time you go to start it. A bad voltage regulator can cause all kinds of odd electrical issues, so if your vehicle seems to have a mind of its own lately, this is very likely the source of your problems.
Worn-out or damaged wiring Damaged wires can cause short circuits, which may ultimately keep your battery from charging properly before you shut the vehicle off. If you’ve ruled out every other potential issue and your battery is still dying overnight, see a mechanic to have them inspect all of your vehicle’s wiring.
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